Re: [SI-LIST] : Differential TDR "Measurements"

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From: Dave Hoover (dave_hoover@yahoo.com)
Date: Wed Apr 26 2000 - 06:39:55 PDT


Patrick,

I asked a TDR Guru from Tek (MikeT) what his
enterpretation of approach 2 is and he sent the
following:

"Dave,

"Approach 1" is the ONLY correct way to measure
differential impedance.

"Approach 2" is NOT differential impedance. It is two
regular or single-ended TDR measurements. I don't
know what "magic of mathematics" is referred to, but I
do know that there is no way to compute true
differential impedance using two regular TDR
measurements. What CAN be done, is to MODEL a change
in single ended impedance and CORRELATE that to a
change in differential impedance. This is called
modeling, and like all modeling, it is only as good as
the model, and the ASSUMPTIONS that go into the
model.

Differential Impedance is DEFINED as two times the odd
mode impedance. Odd mode impedance is DEFINED as the
impedance measured on one of the lines, while the
other line is being driven by an equal amplitude,
opposite polarity TDR pulse. There is NO question
about it, there is exactly one correct way to MEASURE
it.

In my opinion, modeling has a place in printed circuit
board manufacturing - in DESIGN!

Quality Control Test should be about making
independent measurements, that make NO ASSUMPTIONS
about the design."

(Dave speaking here) I wasn't quite sure what you
meant with the approach 2 until I saw MikeT's
response.
If MikeT describes that test correctly, then I
understand it better. Fabricators used this method
for years before we had any way to TDR test
differential signals. Main reason being was we only
had one sampling head. Yes, we could correlate back
to a given reading and lot of PCBs. Correlation HAD
to be done for each design lot (part number). This was
time consuming and awkward. Then decent equipment came
around with multiple sampling heads that allowed us to
really test differential (odd mode). I have to say I'm
in agreement with MikeT's response.

Dave Hoover
(BTW - I co-chair IPC-2141 - Controlled Impedance
Circuit Boards and High Speed Logic Design.)
--- "Zabinski, Patrick J." <zabinski.patrick@mayo.edu>
wrote:
> We're working more and more with differential
> signals,
> and subsequently dealing with more differential
> printed
> circuit boards (PCBs). Over the past few years,
> we've
> had difficulty with several PCB vendors
> trying to obtain a controlled impedance 100 ohm
> differential pair.
>
> The problem generally boils down to "who's
> measurement
> do we believe"? We measure one impedance, while the
> PCB vendor measures another.
>
> We've done some digging, and there appears to be two
> approaches to measuring differential impedance, and
> I'd
> like to hear what folks have to say about them.
>
> Approach 1: inject two signals of opposite polarity,
> one into the true and one into the complement. The
> complement signal is substracted from the true, and
> you read the impedance just like a single-ended
> measurement.
>
> Approach 2: Inject one signal into the true trace
> and
> record its signal. Then, inject a signal into the
> complement
> trace and record its signal. Then, with the magic
> of
> mathematics, compile these two different captured
> signals
> into an effective differential measurement.
>
> The equipment we have in-house uses Approach 1,
> while
> nearly every board vendor we work with uses Approach
> 2.
> Can anyone shed some light into the accuracies,
> sensitivities,
> etc. of these two approaches? Are there cases where
> one
> approach is better/worse than the other?
>
> Thanks,
> Pat
>
> -----
> Pat Zabinski
> ph: 507-284-5936
> Mayo Foundation
> fx: 507-284-9171
> 200 First Street SW
> zabinski.patrick@mayo.edu
> Rochester, MN 55905
> www.mayo.edu/sppdg/sppdg_home_page.html
>
>
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